1. Field of the Invention
The present invention relates to an optical information processing apparatus for recording and/or reproducing information on a recording medium such as light disks and light cards and, more particularly to an optical information processing apparatus having at least either a tracking servomechanism or a focusing servomechanism.
2. Description of the Prior Art
A track for recording information is formed in the form of a concentric circle or spirally on an information recording medium such as light disks. On a light card, a plurality of tracks are formed in parallel in accordance with its shape. When recording or reproducing information on such a recording medium, a light spot is focused on a target track, and it scans the target track while maintaining a focused state.
That is, by means of auto-focusing for focusing the light spot on the target track and auto-tracking control for allowing the light spot to follow up the target track, the information can be recorded on the target track or the information on the target track can be read out.
To perform such an auto-tracking or auto-focusing operation, a change in light distribution of reflected light from the information recording medium or a change in the amount of light itself is detected to determine the amount of change of the spot from the track center position or the focused position. A control signal is then transmitted corresponding to the amount of change to an actuator, a device for shifting an objective lens, in order to shift the objective lens. Then the spot will be returned to the track center position or the focused position.
Assuming the change in light distribution or the change in the amount of light itself generates an error signal in this case, the change in the error signal with respect to the displacement of the spot is as shown in FIG. 1. FIG. 1 is generally called an "S-shaped curve", and it is desired that there should be no offset, that is, the error signal should be 0 when the displacement is 0, and that it should be symmetrical against a+ and- change and be preferably linear.
As conventional detection systems for tracking error signals, there are the push-pull method, the heterodyne method, etc. Also as detection systems for focusing error signals, the astigmatism method, the critical angle method, etc., are known. In any of these systems, reflected light from an information recording medium is guided onto a divided photoelectric conversion element, and an error signal is generated from signals such as a difference, and a sum of detection signals obtained from each element.
In the auto-tracking and auto-focusing systems, of the above conventional example, however, such an offset as shown in FIG. 1 does not exist. However, there are the following problems in obtaining a center-symmetrical S-shaped curve.
(1) A narrow range of dispersion is characteristic for each of the split photoelectric conversion elements and for each element of the electric circuit incidental thereto, and the specification for those parts is severe.
(2) High precision of alignment is desired for each optical system including photoelectric conversion elements.